Keeping it DRY.... where to put shared methods?

My code is starting to have duplicates.... because I don't know where to put methods/functions that need to be accessed by (2) different controllers, and just to get it up quickly I just duplicated it.

So I have a users_controller, and a pages_controller and posts_controller.

On the users page, I have posts being displayed.

On the homepage, I also have posts being displayed.

Where should I put shared/common code so that the users_controller and pages_controller and posts_controller can all have access to ithese methods?

Right now, I have duplicated def methods in some controllers, and also some duplicate private methods in each controller.

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On a related note, how can I access methods defined in one controller from another controller?